Myostatin is a protein growth factor that regulates muscle growth. It is present in all mammals, but in some particular cattle breeds the regulator is ‘switched off’, or deleted, so muscle can continue to develop unchecked. Not all breeds demonstrate the same variant of the gene and there is also an interaction with other genes that is not the same in all breeds. This complicates the way in which the deletion is expressed, so that not all cattle carrying it are visibly muscled.

All South Devon bulls being registered as pedigree must be DNA tested for myostatin status, which is proving essential to assess the likely mating outcome of a particular sire and dam. The status is reported as: ‘0’ – no myostatin deletion; ‘1’ – one copy inherited from either sire or dam; or ‘2’ – two copies, one inherited from sire and one from dam.

Currently in the UK, Igenity test for nine different myostatin variants. Six of these variants are so-called disruptive, and three are so-called mis-sense. The most common variant in the South Devon is the disruptive ‘nt821’; and also present in some South Devons is the mis-sense ‘F94L’ variant.

Results of a study into the effects of the ‘nt821’ variant by the Roslin Institute (2006) reported little difference in calving score and growth in ‘0’ and ‘1’ status animals. But in animals with two copies of the ‘nt821’ variant, the study showed an increase in birth weight of about 2 kg linked to an association with calving difficulty. In terms of growth animals with two copies of the gene showed a slowdown as they grew older, so that by 36 weeks their weight was about 30 kg lighter than the ‘0’s and ‘1’s. As regards carcase traits, myostatin deletion was associated with an increase in hot carcase weight and conformation score, and a reduction in fat score and taste.

Another study conducted by the University of Adelaide (2008) on the influence of the ‘F94L’ variant reported a 19% increase in carcase prime cuts, an overall 7% increase in beef yield, more tenderness, 20% less intramuscular fat, and 30% less external fat cover. It is significant that the ‘F94L’ variant is not associated with higher birth weights, and research shows that muscle growth happens following birth, not during gestation.